Components and structures which are subject to dynamic loading may be monitored for fatigue damage. This damage is sometimes evidenced by surface breaking cracks. Monitoring for fatigue damage is often important to maintain safety for people and equipment which use or operate in or on a multitude of engineered structures such as bridges, buildings, aircraft and rail transport systems. In the rail transport environment for example, rolling contact fatigue (RCF) is a major cause of rail track and rail wheel damage. There are several types of RCF damage including cracking, spalling and squats. Cracking is typically manifested by a continuous band of fine cracks that initiate in the gauge corner of a head of the rail. Spalling is characterised by small pieces of the rail head breaking away from the surface where RCF cracks have coalesced. Squats are subsurface voids that are not visible on the rail surface but become visible when the rail surface is milled away.
In the rail environment, RCF is a result of repeated overstressing of the surface or subsurface material of a rail track or wheel as a result of repeated wheel-rail contact cycles.
It is common in the maintenance of rail tracks to periodically grind or mill the running surface of a rail track to remove or at least decrease the depth of cracks and defects in the track surface to thereby extend the service life of the rail track and avoid catastrophic failure. To avoid grinding and milling the entire rail track in a rail network various non-destructive testing techniques are used to detect the existence of cracks. Examples of non-destructive testing techniques include ultrasonic testing, magnetic testing, eddy current systems, and differential pressure crack monitoring systems. These techniques are not limited to use in a rail environment and are used for testing of other load bearing components or structures made of a magnetisable material. In many situations, the mere existence of a crack is not in itself of major concern. It is other characteristics of the crack that trigger maintenance or corrective procedures for a load bearing component or structure. One of these characteristics is crack depth. For example, the existence of multiple shallow surface cracks may not require any action whereas a single deep crack may require immediate action.